The Rock Cycle: Igneous Rocks

What is a rock? An aggregate of minerals 3 types of rock Rock cycle Igneous Sedimentary Metamorphic Rock cycle

Igneous Rocks Rocks created when molten rock cools and crystallizes Two types Extrusive (volcanic) Erupted and cooled quickly at the surface Intrusive (plutonic) Cooled slowly beneath the surface of the earth

Extrusive Igneous Rocks

Extrusive Igneous Rocks Fine grained basalt Occur when lava cools quickly on the surface Fine-grained textures common Cools quickly, crystals have little time to cool Porphyry—large crystals formed at depth prior to emplacement on surface Glassy obsidian Porphyritic texture

Intrusive Rocks Occur when magma cools within the crust Granite: a common intrusive rock Occur when magma cools within the crust Medium to course grained texture Crystals can generally be identified without the aid of a magnifier Large crystals are the result of slow cooling over thousands/millions of years Diorite: another common intrusive rock

“Can I see your I.D., please?” Rocks are identified by Textures Mineral compositions Ex: We have a rock that is composed of feldspar, quartz and hornblende—could be granite or rhyolite Texture will help us differentiate between the two Fine grained: rhyolite Course grained: granite

More on classification… Rhyolite: felsic Felsic Light colored—generally composed of light colored minerals mafic Generally composed of Fe/Mg—dark colored minerals Intermediate rocks—composition between felsic and mafic “Salt and pepper” Gabbro: mafic Diorite: intermediate

More on Classification peridotite Ultramafic rocks: Very low Si Dark colored, and heavy for their size Often contain dark pyroxene, and large amounts of olivine—a granular-looking, green mineral dunite

How do magmas form? Si rich minerals—feldspar, quartz, etc.. Have low melting points Melting point increases with decreasing Si content

Partial Melting Melts of different compositions can be generated by melting the same rock several times at different temperatures This is why not all igneous rocks look the same Can also occur as magmas cool

How does rock melt? Increase temp Temp increases 30°C for every kilometer of depth At depths of 100 to 350 km, the temp is so high that some areas of melt begin to form There are other ways; well talk about them later